A metrology tool for measuring the positions of structures on masks or substrates, respectively, used in the production of semiconductors, is disclosed in the presentation manuscript “Pattern Placement Metrology For Mask Making” by Dr. Carola Biasing. The presentation was given at the conference Semicon, Education Program, in Geneva on Mar. 31, 1998. In this presentation manuscript the fundamentals of a metrology tool are described in detail. For the details of operation and for the configuration of a metrology tool explicit reference to FIG. 1 of the patent application is made.
The German patent DE 103 216 80 D4 discloses a method for determining the quality of a pellicle on a frame located on a mask for the protection of the structures formed on the mask. The structures are in particular protected against contamination with microscopic particles by the foil. The mask is inserted into the exposure tool together with the pellicle, by sending a beam through the pellicle and the mask the quality of the pellicle or of the foil tightened on the frame, respectively, can be inferred from the resulting diffraction.
The U.S. Pat. No. 5,853,927 discloses a method for aligning a mask in a photolithographic process. The mask carries a frame with a pellicle for the protection of the structures below. Already with the naked eye or by using a microscope a user can identify bad alignment or attachment of the pellicle frame with respect to the mask. For this purpose a number of alignment marks are provided on the mask, by which the alignment or the correct attachment of the pellicle frame with respect to the mask can be checked and corrected, respectively.
The Japanese patent application JP 10073407 discloses a metrology tool also measuring the positions of structures on a mask protected by a pellicle frame with a foil. In the apparatus proposed here, the bending or curvature of the foil or pellicle, respectively, is taken into account in the determination of the positions of the structures on the mask. With the apparatus proposed in the patent application it is possible to determine the curvature of the pellicle tightened in the frame, and to take it into account when measuring the positions of the structures on the mask, if indicated.
The Japanese patent application JP 2005 292197 discloses an apparatus for measuring the position of attachment of the pellicle frame on the mask. The position of the pellicle frame on the mask is determined by a contactless measurement apparatus.
The Japanese patent application JP 4133061 discloses an apparatus for determining the planarity of a photomask. A laser beam is sent through the mask. Behind the mask a mirror is located, so that from resulting interference fringes due to thickness variations of the mask the planarity of the mask can be inferred.
The Japanese patent application JP 5061187 also discloses an apparatus for the inspection of the planarity of a mask provided with a pellicle frame. The surface of the mask is illuminated with a laser beam through the pellicle. The objective and the detection system for the remitted light beam are adjusted in such a way that the light from the position to be detected on the mask reaches a pre-defined position through the pellicle. The height of the position hit by the laser is determined based on the adjusted and measured position. Based on plural measurement points distributed on the surface of the mask, the planarity of the mask eventually can be inferred.
In determining the positions of structures on the surface of a mask with a metrology tool the bending of a mask is taken into account. Up to now it has not been considered that even a pellicle frame attached to the mask and provided with a pellicle affects the bending behaviour of a mask. In measurements up to now the measurement values determined with a metrology tool were only corrected with a theoretical model resulting from the bending of a mask not provided with a pellicle frame. The bending of the mask therein was calculated with a system in which the mask is in contact with three support points. Based on the calculated model the position of structures on the mask measured with the metrology tool were corrected. The measurement of positions of structures on the surface of masks provided with a pellicle frame is of growing importance, as the relevance of overlay between masks is increasing. Attaching the pellicle frame to the completely structured mask leads to an additional stress inside the mask, which, relative to an ideal flat mask effects a position shift of the structure to be measured of about 1 nm to 3 nm. With the present 6-inch standard masks a bending model is used which does not take into account the physical properties and the physical behaviour of a mask with a pellicle frame.